The plasma waveguide switch (PWS) is a pulsed high power microwave switching device consisting of a cathode, anode, and waveguide grid all hermetically sealed and filled with an appropriate gas. The waveguide broadwalls are perforated to allow passage of a plasma discharge between cathode and anode. The waveguide grid is usually operated at ground potential with cathode pulsed at negative potential to initiate the discharge between cathode and grid. A secondary grid can be used at an intermediate negative pulsed voltage between the cathode and grounded waveguide grid to initiate the discharge. A pulse forming network connected to the anode then discharges through the device creating a dense plasma in the waveguide. If the density is high enough, this plasma is opaque to electromagnetic waves at microwave frequencies and forms an effective pulsed microwave waveguide switch. The higher the frequency the higher is the plasma density required.
At millimeter frequencies a plasma focusing cone is used between the cathode and the waveguide grid to increase plasma density. The focusing cone is then operated at an intermediate negative pulsed voltage between the cathode and grounded waveguide grid. This pulsed voltage then initiates the plasma discharge.
One of the most important uses of the PWS is in pulse shaping. By proper pulse timing the PWS can be made to fire at any instant during passage of the RF pulse through the PWS. Since the formation of the plasma is in the order of a nanosecond, the reflected chopped RF pulse will have a rise time of approximately 1 to 2 nanoseconds while the transmitted RF pulse will have a fall time of approximately 1 to 2 nanoseconds. By using 2 PWS's in a microwave system, in which PWS No. 1 chops the front of the pulse and PWS No. 2 chops the back of the pulse, an RF pulse can be chopped twice to form a pulse of any width (as short as a few nanoseconds if desired) and with rise and fall time of the order of 1 to 2 nanoseconds.